As a conventional method of estimating a magnetic pole position, widely used is a method in which an induced voltage that is proportional to a motor speed is calculated from a motor input voltage and a current, and the speed is estimated, such as that reported in “Adaptive Current Control Method for Brushless DC motor with Function of Parameter Identification” IEEJ Transactions on Industry Applications, Vol. 108 No. 12, 1988. Also known is “Zero Speed Torque Control of Sensorless Salient-Pole Synchronous Motor” 1996 National Convention of IEEJ Industry Applications Society No. 170. In this technique, an AC signal is superimposed on a voltage command value, and an estimated current is FFT analyzed to estimate the rotational speed of a motor and a magnetic pole position. However, a method which estimates the speed and position of a rotor on the basis of an induced voltage of a motor operates with sufficient accuracy in a high speed region, but cannot perform correct estimation at a very low speed from which little information of the induced voltage is obtained.
Therefore, several methods have been proposed in which an AC signal that has no relation to a driving frequency, and that is used for sensing is injected into a motor, and a rotor position is estimated from relationships between the voltage and the current. However, a special signal generator is necessary in order to inject such a sensing signal, thereby causing a problem in that the control is complicated. Other methods in which a special sensing signal is not injected and a magnetic pole position is estimated by using high frequencies of an inverter output or currents of carrier frequency components are reported in “Position Sensorless IPM Motor Drive System Using Position Estimation Method Based on Saliency” IEEJ Transactions on Industry Applications, Vol. 118 No. 5, 1998, and “Carrier Frequency Component Method for Position Sensorless Control of IPM Motor in Lower Speed Range” IEEJ Transactions on Industry Applications, Vol. 120 No. 2, 2000. The former method is characterized in that an inductance is calculated from high-frequency currents generated by output voltage high frequencies of a PWM inverter, and the position is estimated on the basis of the inductance.
The latter method is characterized in that a phase difference of 120 deg. is produced in PWM inverter carrier signals between two of three or UVW phases to generate voltages and currents of carrier frequency components other than the driving frequency, and the position is estimated by using only the carrier frequency component currents based on the assumption that a voltage during the carrier period is constant.
In IECON' 01 (Proc. of the 27th Annual Conference of the IEEE Industrial Electronics Society pp. 1435–1440) “Novel Rotor Position Extraction based on Carrier Frequency Component Method (CFCM) using Two Reference-frames for IPM drivers”, and Patent Reference 1 which is a prior patent application in the name of the same applicant, in order to further facilitate practical use of the latter method, the problem in that synchronization between plural current estimation timings in a carrier period and position calculation is complicated was solved by a method in which, with respect to a high-frequency current converted to four axes, a moving average value is used in each of the axes as described later.
[Patent Reference 1]
Publication of Japanese Patent Application No. 2001-238060
Specifically, FIG. 6 is a block diagram of a conventional apparatus for detecting a magnetic pole position. In the magnetic pole position detector 62, three-phase high-frequency currents output from a band-pass filter are converted by a coordinate converter 64 to the α-axis, the β-axis, the α′-axis, and the β′-axis. Peak values of the four converted outputs are averaged by an absolute value calculator 65 and a low-pass filter 66. On the bases of proportional relationships with inductances of the axes, a pole position calculator 67 calculates tan(2Δθ) to obtain Δθ, thereby calculating the magnetic pole position.